This invention relates in general to highway or airport pavements of concrete.
Portland cement concrete pavements of highways or airports have long service life and high strength, and are used as a main kind of pavements. But they have significant problems. Plain concrete pavements have many joints in the pavement slabs and the joints bring about cracking, edge failure, faulting, uneven, and/or pumping problems. Simply reinforced concrete pavements have less joints but some minute cracks that cannot be prevented by the reinforcement; the cracks bring about edge failure, water invasion, and other cracks. Continuously reinforced concrete pavements can eliminate the joints, but get more minute cracks and cost considerably more.
To solve these problems, prestressed Portland cement concrete pavements (pcp) were and are developed. The longitudinal compressive prestress in pcp slabs can offset the tensile stress due to temperature drop so the transverse joints in the slabs can be reduced or eliminated, offset the tensile stress due to a vehicle load so the slab thickness can be reduced, and close slab cracks caused by various accident reasons so the slab integrity can be kept. Furthermore, because pcp joints are significantly fewer than plain or reinforced concrete pavement joints, pcp do not require their subbases as strong and water-resistant as the subbases of plain or reinforced concrete pavements, pcp service life is longer, and pcp maintenance is less. Therefore, pcp are expected to be more cost-effective than plain or reinforced concrete pavements.
But pcp fail to prevail in road construction, although they appeared many years ago. The reason is that their structures and constructions are complicated and expensive.
Pcp are divided into two categories: pcp with tendons and tendonless pcp. The former may be divided further into pretensioned and post-tensioned pcp. While the pcp with tendons use steel tendons or other tensile materials to provide compressive prestress for the concrete slabs, the tendonless pcp use abutments to provide the prestress.
The pretensioned pcp are built similarly to the one in general pretensioned concrete production. The steel tendons or wires are stretched and anchored to temporary or permanent abutments in situ. The concrete is then cast around the prestressed tendons. After the concrete has gained sufficient strength, the tendons are released from the abutments. The bond between the tendons and concrete prevents the tendons from shortening, which results in the compression in the concrete. On the contrary, in the post-tensioned pcp construction concrete is first cast in place with embedded unbound tendons or bonded hollow ducts. After the concrete has attained sufficient strength, the tendons are stressed and anchored at the ends of the pavement slab. The tendons may be unbonded, being coated with grease, or bonded, being grouted between tendons and ducts. In the tendonless pcp construction, a series of gap-separated slabs is built between permanent abutments. Flat jacks set in the gaps provide compression to the concrete slabs. Then the gaps are filled with concrete and the jacks are removed.
The pcp with tendons cost much in tensile material, and the auxiliaries for and the operations of the material. The tendonless pcp require expensive abutments, and lose more prestress. Both of the pcp with tendons and tendonless pcp involve complicated prestressing operation and second-time concrete casting to fill up gaps that are needed for prestressing.
The present invention is a low-cost, high-quality pcp. It does not require longitudinal tendons, its construction is simple and fast, and its prestress is controlled in a predetermined range.
This new pcp comprises apparatus embraced in the pavement slabs for longitudinally prestressing the slabs, comprises slab abutments for resisting the compression, comprises restraints connecting the slabs and the pavement sub-base/grade for preventing the slab buckling, and comprises hydraulic accumulators communicating with the apparatus for controlling the prestress level in the pavement service. If desired, the new pcp also comprises prestressing devices and bars for transversely prestressing the slabs. The apparatus, devices, reinforcement in the slab abutments, prestressing bars, and restraints are all pre-installed or pre-built in/on the pavement support base(subbase and/or subgrade). One pass of a conventional slip-form paver completes slab cast and connection of the slabs and the facilities. Prestressing operation is mainly pumping fluid and grout into the apparatus and devices.
The apparatus crossing the pavement slabs comprise slab slit-forming means and slit-expanding means for compressing the pavement slabs. The slit-forming and expanding means comprise hydraulic cylinders and pistons, or sealed thin flexible tubes which expand the slits to provide the slab prestress in the pavement construction, and which contract or expand the slits to regulate the slab prestress in the pavement service.
The slab abutment utilizes be friction and interlock between the abutment slabs and the support base to resist the prestress load. Since the abutment slabs belong to the general pavement slabs and their main cost is included in the pavement slab cost, the abutments are economical. Other man-made or natural objects being near the pavement and having horizontal resistance may be utilized as abutments. If a structure is designed to possess an abutment function as well is its own function, the sum of the structure cost and the pavement cost is less than the sum of the corresponding normal structure cost and an independent pavement cost.
The restraints using mechanism or vacuum are able to restrain upward deflection of any point of the pavement slabs for preventing buckling of the long slabs, but allow the tangential displacement of the slabs for avoiding obstruction of the prestress transmission.
The short-life friction-reducing medium is applied, because the unfavorable functions of the friction are only in a short period. The medium consists of layers of paper and lubricating jelly that are economical.
The accumulators comprise fluid and compressed air for damping a pressure rise in the apparatus due to temperature increase, and compensating for the pressure drop due to temperature decrease, abutment displacement, and/or concrete shrinkage and creep. According to a hydraulic theory, provided there is a sufficient ratio of compressed-air volume to fluid volume differential in the accumulator, the pressure differential in the apparatus, hence the prestress differential in the pavement slabs, can keep in a sufficiently small range.
Although the present pcp mainly relates to longitudinally tendonless pcp, it can be utilized in other kinds of pcp. This new pcp is suitable to pavements of concrete and pavements of other materials having sufficient compressive strengths. Although the new pcp in general relates to highway and airport pavements, obviously it may extend to parking lots, bridge decks, warehouse floors, etc.
Accordingly, an object of the present invention is to provide a prestressed pavement (prestressed Portland cement concrete pavement or other kinds of prestressed pavement) without longitudinal tendons.
Another object of the invention is to provide a prestressed pavement with embedded prestressing apparatus for easily and quickly prestressing.
Yet another object of the invention is to provide a prestressed pavement that is built by one-time concrete casting and a continuously paving machine.
Yet another object of the invention is to provide a prestressed pavement that favors step and/or delayed prestressing.
Still another object of the invention is to provide a prestressed pavement whose prestress can be measured, controlled, regulated, and/or supplemented during its service.
Still another object of the invention is to provide a prestressed pavement whose joints can transfer most all longitudinal force, shear, and moment from one slab to another.
Still another object of the invention is to provide a prestressed pavement on which the later overlay can easily get prestress.
Still another object of the invention is to provide a prestressed pavement whose prestress can be temporarily removed for rehabilitation.
These and other objects of the present invention will become apparent in the DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT.